The present invention relates to medical leads and microelectrode recording systems used for functional neurosurgical procedures.
Deep brain stimulation (DBS) is being increasingly accepted as a viable treatment modality. In particular DBS applied to the thalamus for treatment of tremor was approved by the FDA in 1997. Subsequently, other diseases, such as Parkinson's Disease, dystonia, and chronic pain, among others, have been identified as candidates for treatment with deep brain stimulation.
The term “stylet,” as used in this disclosure, is an implement inserted into the lumen of a stimulating lead to stiffen the lead and to facilitate its insertion into the target tissue. The term “rod,” as used in this disclosure, is an implement that is placed inside a cannula to provide support to the cannula, while it is inserted into target tissue. The term “microelectrode” refers to a recording electrode which can be essentially a wire which has at least the distal portion of the wire uninsulated to receive electrical signals from the recorded tissue. The term “macroelectrode” will refer to a stimulating electrode and parts connected to the electrode, which macroelectrode is intended as a temporary test electrode to perform macrostimulation. Macrostimulation involves stimulating many cells at once. The term “lead,” as used hereinafter, will specifically refer only to a chronically implantable stimulation electrode, including parts connected to the electrode. The electrode portion of the lead is that portion which is in electrical contact with tissue. The term “tract” refers to an individual pathway formed in tissue, for example, by inserting a microelectrode, a macroelectrode, a lead or an associated cannula into that tissue.
Implantation of a lead for deep brain stimulation generally involves the following preliminary steps: (a) anatomical mapping and (b) physiological mapping. Anatomical mapping involves mapping segments of an individual's brain anatomy using non-invasive imaging techniques such as magnetic resonance imaging (MRI) and computed axial tomography (CAT) scans. Physiological mapping involves localizing the brain site to be stimulated. Step (b) can be further divided into: (i) preliminarily identifying a promising brain site by recording individual cell activity with a microelectrode and (ii) confirming physiological stimulation efficacy of that site by performing a test stimulation with a macroelectrode.
Microelectrode recording is generally performed with a small diameter electrode with a relatively small surface area optimal for recording cell activity. The microelectrode may be essentially a wire which has at least the distal portion uninsulated to receive electrical signals. The rest of the body or wire of the microelectrode may be insulated. The microelectrode functions as a probe to locate a promising brain site. Since a number of attempts may be required to locate the precise target site, it is desirable that the microelectrode be as small as possible to minimize trauma when the microelectrode is placed into the brain, in some cases, multiple times.
Once a brain site has been identified, a macroelectrode is used to test that the applied stimulation has the intended therapeutic effect. A macroelectrode is a temporary stimulating electrode and is not intended to be chronically implanted. Because macrostimulation involves stimulating many cells at once, an optimal electrode for macrostimulation requires a larger surface area compared to a microelectrode, which merely records electrical activity from a single cell or a few cells. For this reason, the conductive electrode surface of a macroelectrode is generally larger than the conductive electrode surface of a microelectrode. The macroelectrode can be retraced into the same brain site identified with microelectrode cell recordings.
Test stimulation with the macroelectrode may need to be performed in a number of tracts in order to localize the site which provides the proper physiological effect. Because the macroelectrode may need to be repeatedly inserted into the brain, the macroelectrode must be durable, stiff and resistant to buckling. The macroelectrode can be made from a sterilizable material.
Once macrostimulation confirms that stimulation at the brain site provides the intended physiological effect, the macroelectrode is withdrawn from the brain and a DBS lead is permanently implanted at the exact site.
Keeping in mind the above general steps, a conventional procedure for carrying out DBS may involve the following detailed steps: (1) place a stereotactic frame on the subject, which stereotactic frame is a device temporarily mounted on the head to assist in guiding the lead system into the brain; (2) perform MRI or equivalent imaging of the subject with the stereotactic frame; (3) identify a theoretical target using a planning software; (4) place the subject with the stereotactic frame in a head rest; (5) using scalp clips, cut the subject's skin flap in the head, exposing the working surface area of the cranium; (6) place the stereotactic arc with target coordinate settings and identify the location on skull for creation of a burr hole; (7) remove the arc and drill a burr hole in the patient's skull; (8) place the base of the lead anchor; and (9) with the microelectrode recording drive attached, and with an appropriate stereotactic frame adaptor inserted into the instrument guide, place the stereotactic arc.
Next, (10) advance a microelectrode cannula and an insertion rod into the brain until they are approximately 25 mm above the target; (11) remove the insertion rod, leaving the cannula in place; (12) insert a recording microelectrode such that the tip of the microelectrode is flush with the tip of the microelectrode cannula; (13) connect the connector pin of the recording microelectrode to a microelectrode recording system; (14) starting approximately 25 mm above the target, advance the microelectrode into a recording tract at the specified rate using the microdrive; and (15) if the target is identified, proceed to step 16. If the target is not identified, proceed with the following: (17) using the recording results and pre-operative imaging, (a) determine a new set of coordinates for the theoretical target; (b) disconnect the recording microelectrode from the microelectrode recording system; (c) remove the recording microelectrode cannula and recording microelectrode; and (d) adjust the coordinates of the stereotactic frame. Then, continue at step 10, above.
Next, (16) remove the recording microelectrode cannula and recording microelectrode; (17) insert a macroelectrode insertion cannula and rod until they are approximately 25 mm above the target; (18) remove the insertion rod, leaving the macroelectrode insertion cannula in place; (19) insert a stimulating macroelectrode, and advance it to the target stimulation site identified by the recording microelectrode; (20) using macrostimulation, simulate the stimulation of the chronic DBS lead to ensure proper physiological response; (21) remove the macroelectrode and cannula; (22) insert a DBS lead insertion cannula and an insertion rod, and advance both to approximately 25 mm above the stimulation site; (23) remove the insertion rod; (24) insert a DBS lead, with stylet, through the insertion cannula, and advance the lead/stylet to the stimulation site; (25) electrically connect the lead to a trial stimulator; and (26) perform the desired stimulation and measurements using any one or combination of four electrodes on the DBS lead.
Next, (27) if the results are favorable, proceed to step 28. If the results are not favorable, proceed with the following: (a) using the macrostimulation results and microelectrode recording results, as well as pre-operative imaging, determine a new set of coordinates for the theoretical target; (b) remove the lead and stylet; (c) remove the insertion cannula; (d) adjust the coordinates of the stereotactic frame; and (e) continue at step 10, above.
Next, (28) remove the stylet, followed by the insertion cannula; (29) using macrostimulation, verify that micro-dislodgement of the DBS lead has not occurred; and, finally, (30) lock the DBS lead in the lead anchor.
Some physicians might use additional steps, fewer steps, or perform the steps in a different order.
There are a number of commercially available microelectrode recording (“MER”) systems used for deep brain stimulation. Such a system includes apparatuses for holding the microelectrodes in place and electronics that connect to the microelectrodes to enable cell recordings. MER systems are sold by Alpha Omega Engineering (Nazareth, Israel), Axon (Union City, Calif.), Atlanta Research Group (Atlanta, Ga.), and Microrecording Consultants (Pasadena, Calif.). The Alpha Omega and Axon systems appear to be among the most popular with functional neurosurgeons. None of these companies manufacture their own microelectrodes, although they may provide a microelectrode as part of the MER system package. The Fred Haer Corporation (FHC) markets a popular microelectrode which is sometimes provided in the MER system package.
The Alpha Omega Engineering MER system permits the neurosurgeon to simultaneously record “five-electrodes-at-a-time” recordings. In this approach all five of the microelectrodes are advanced into the brain at the same time and at the same speed. This presents obvious advantages. The set-up time may be proportionately cut, since the chance of locating a good stimulation site theoretically increases by five times. A disadvantage presented is that because the microelectrodes are placed relatively close to each other, two of these electrodes could “capture” a blood vessel between the electrodes, puncturing the vessel and possibly leading to intracranial bleeding. In contrast, when a single microelectrode is used, the blood vessel can often escape injury because the vessel can deflect away from the microelectrode or vice-versa. Thus, some neurosurgeons choose to use the Alpha Omega MER system with only a single microdrive, advancing one microelectrode at a time until a suitable placement site is found. Other neurosurgeons have used the Alpha Omega system with two independent microdrives, which provides the flexibility of recording independently from two tracts.
Other neurosurgeons use the Axon system, which can manually advance only one microelectrode at time. Some neurosurgeons average 4 to 5 microelectrode recording tracts to identify a suitable brain site. Other neurosurgeons only record from one recording tract, which cuts surgery duration, but which may not locate an optimal stimulation site. Without optimal electrode placement, the DBS lead may need to be stimulated at higher currents, which can cause the device battery to be drained more quickly. In addition, use of higher currents can increase the risk of undesirable side effects such as dysarthria (slurred speech) and abulia (an abnormal inability to make decisions or to act).
Each of these MER systems apply the conventional surgical procedure of using the microelectrode to find the target brain site, withdrawing the microelectrode, then placing a macroelectrode, followed by placing a DBS lead or alternatively, placing a DBS lead directly without using a macroelectrode. These conventional surgical procedures are far from ideal. The number of steps lengthen the surgical procedure and increase the risk for post-operative infection. In addition, having to retrace the microelectrode pathway and the macroelectrode pathway to place the DBS lead substantially increases the chances for misalignment and misplacement because each of these steps require use of a separate introduction cannula. Morever, the use of at least three cannulas in the procedure can increase surgical duration and operative risk, simply from the number of objects inserted into the brain. In addition, retracing the pathway of the microelectrode and the macroelectrode as preliminary steps to placing the permanent DBS lead is fraught with misalignment/misplacement problems because the introduction cannulas may not trace the exact pathways desired. When there is a missed placement of a DBS lead, the DBS lead and stylet may have to be scrapped.
Accordingly, there is a need for a DBS lead/microelectrode system which is compatible with the available MER systems and the various methods of employing these recording systems, which DBS lead/microelectrode system eliminates surgical steps and reduces surgical duration, reduces operative risk, and improves the accuracy of placing the permanent stimulation DBS lead to provide optimal physiological therapy.